A nuclear fuel aggregate, which has been burnt and is no longer used in its terminal point of a nuclear fuel cycle, is referred to as a spent fuel aggregate. The spent fuel aggregate, which contains high radioactive substances such as FP, needs to be thermally cooled off so that it is cooled off for a predetermined period (for three to six months) in a cooling pit in a nuclear power plant. Thereafter, this is housed in a cask that is a shielding container, and transported by a truck or a ship to a recycling facility where it is stored. Upon housing the spent fuel aggregates in the cask, a holding frame having a lattice shape in its cross-section, called a basket, is used. The spent fuel aggregates are inserted into cells that are a plurality of housing spaces formed in the basket, one by one, thus, it is possible to ensure an appropriate holding strength against vibration, etc. during the transportation.
With respect to conventional examples of such a cask, various types thereof have been proposed in “Atomic eye” (issued on Apr. 1, 1998, Nikkan Kogyo Publishing Production) and Japanese Patent Application Laid-Open No. 62-242725. The following description will discuss a cask that forms a premise upon developing the present invention. However, the cask is shown for convenience of explanation, and is not necessarily related to the conventionally known and used device.
FIG. 23 is a perspective view that shows one example of a cask. FIG. 24 is a cross-sectional view in the radial direction of the cask shown in FIG. 23. A cask 500 is constituted by a cylinder-shaped trunk main body 501, a resin portion 502 placed on the outer circumference of the trunk main body 501, an outer cylinder 503, a bottom 504 and a lid section 505. The trunk main body 501 and the bottom 504 are forged products of carbon steel that is a γ ray-shielding substance. Moreover, the lid section 505 is constituted by a primary lid 506 and a secondary lid 507 made of stainless steel. The trunk main body 501 and the bottom 504 are joined to each other through butt welding.
The primary lid 506 and the secondary lid 507 are secured to the trunk main body 501 with stainless bolts. A hollow O-ring made of metal to which an aluminum coating, etc. is applied is interpolated between the lid section 505 and the trunk main body 501 so as to maintain the inside thereof in an air-tight state. Trunnions 513 which suspends the cask 500 are placed on both of the sides of a cask main body 512 (one of them is not shown). Moreover, buffer members 514 in which timber, etc. is sealed as a buffer member are attached to both of the ends of the cask main body 512 (one of them is not shown).
A plurality of inner fins 508 which allows heat conduction are placed between the trunk main body 501 and the outer cylinder 503. The inner fins 508 are made of copper as their material in order to increase the efficiency of heat conductivity. Resin 502 is injected into a space formed by the inner fins 508 in a fluid state, and solidified and formed through a thermo-curing reaction, etc. A basket 509 has a construction formed by collecting 69 square pipes 510 into a bundle as shown in FIG. 23, and is inserted into a cavity 511 of the trunk main body 501 in a fixed state. The square pipe 510 is made of an aluminum alloy in which neutron-absorbing member (boron, B) is mixed so as to prevent the inserted spent fuel aggregate from reaching the criticality. Moreover, each housing space formed by each square pipe 510 is referred to as a cell 515, and each cell 515 can house one spent fuel aggregate. Trunnions 513 which suspends the cask 500 are placed on both of the sides of the cask main body 512 (one of them is not shown). Moreover, buffer members 514 in which timber, etc. is sealed as a buffer member are attached to both of the ends of the cask main body 512 (one of them is not shown).
A basket that has been used for a conventional radioactive substance storing container such as a cask and a canister is constituted by combining side faces of a plurality of square pipes with each other, therefore, in order to ensure a sufficient strength at the time of falling down, it is necessary to increase the plate thickness of the square pipe. For example, when a cask horizontally falls down, the load of the spent fuel aggregate is concentrated on the face end portions of each square pipe, thus, it is necessary to provide a thickness that can withstand this impact force. Moreover, since the basket needs to have a function to prevent the inserted spent fuel aggregate from reaching the criticality, the square pipe used for the basket is made of an aluminum alloy in which boron (B) is mixed as a neutron absorbing material. In order to provide this criticality preventive function, the square pipe for the basket needs to have a certain degree of thickness. For this reason, the outer shape dimension of the entire basket tends to become large, resulting in a greater mass in the entire basket.
Moreover, in order to protect the cask main body from an accident such as falling down during transportation of the cask, the buffer members 514 (one of which is omitted from the Figure) are attached to both of the ends of the cask (see FIG. 23). The impact at the time of falling of the cask is buffered with the buffer members 514 being crushed. In this case, the margin of crushing in the radial direction, which is used for buffering the impact at the time of horizontal falling down, is ensured by increasing the diameter of the buffer members 514, however, when land transportation is taken into consideration, the diameter of the buffer member 514 can not be increased unduly. When the outer diameter of the cask main body is reduced, the resulting space can be used as the margin of crushing in the buffer member 514, thereby making it possible to reduce the outer diameter of the buffer member 514.